Radio frequency (RF) spectrum is a scarce resource for wireless or mobile communication systems. Wireless communication systems have been used to provide communication of voice, data, and the like for a number of users. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), and other multiple access techniques. In a cellular or personal communications systems environment, an increasing number of users need to be simultaneously serviced while also attempting to avoid interference among such users.
One attempt at addressing dwindling communication spectral resources is the application of multiple-input multiple-output (MIMO) techniques to achieve desirable spectral efficiencies. Recent research has shown that a rich scattering environment is capable of significant communication capacity due to multipath diversity inherent in the rich scattering environment. MIMO techniques have been used in communication systems that generally employ multiple transmit antennas and multiple receive antenna for transmission of multiple independent data streams. These data streams may be transmitted to one or more terminals. For example, a multiple access communication system having a base station with multiple antennas may concurrently communicate with a number of terminals via a channel matrix.
Most forms of MIMO systems require a reliable estimate of the channel matrix for reception of communication signals. Some MIMO systems use successive interference cancellation techniques to remove effects from undesired transmit channels. One method for channel estimation is the use of training preambles that consist of known data at a MIMO transmitter and receiver. The channel is estimated periodically when a training period occurs. The computed channel estimate is then used without modification for following communication. For example, one signal processing technique that may be used with MIMO communication systems is an orthogonal frequency division multiplexing (OFDM) technique. OFDM distributes data over a large number of carriers that are each spaced apart by frequencies that provide the orthogonality. In this example, the channel estimate is used for OFDM frames following the training preamble until a new training period updates the channel estimate.
The training preambles or sequences help to capture a current channel estimate at a specific point in time. Mobile communication systems exhibit a time and frequency selective channel. Under mobile conditions where the channel is dynamically changing, the channel estimate for each OFDM frame typically incurs some error that may result in the degradation of the MIMO communication system. If only training preambles are employed, the number of applicable OFDM frames that follow the training preambles is generally lesser because the channel estimate may become “stale” within a relatively short amount of time. Communication algorithms such as MIMO generally require knowledge of the communication channel to perform well. Channel tracking may be employed to assist in improving OFDM-MIMO performance.
Pilot based tracking algorithms are common, but share in overhead loss with training preambles. A common difficulty associated with tracking an OFDM-MIMO channel is that the multiple transmit signals create self-interference at a receiver. One approach to minimize this interference is the application of successive interference cancellation to clean-up a received signal. One method used to track a communication channel is decision directed (DD) channel estimation. DD algorithms typically require little to no communication overhead since each subcarrier is treated as its own pilot to obtain a high-resolution channel estimate. However, bit errors tend to degrade the performance of such systems using DD channel estimation.
In view of the foregoing, it is desirable to provide a channel tracker for a time-frequency channel having improved communication channel estimation. In addition, it is desirable to provide a receiver having communication channel tracking to track a time-frequency channel of advance communication algorithms. Furthermore, additional desirable features provided by the invention will become apparent to one skilled in the art from the drawings, foregoing background, following detailed description, and appended claims.